Phase Behavior and Thermodynamic Model Parameters in Simulations Of
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www.nature.com/scientificreports OPEN Phase Behavior and Thermodynamic Model Parameters in Simulations of Extractive Received: 4 April 2017 Accepted: 20 July 2017 Distillation for Azeotrope Published: xx xx xxxx Separation Min Li, Xicai Xu, Xin Li, Kang Ma, Bin Qin, Zhaoyou Zhu & Yinglong Wang Extractive distillation (ED) processes for separating ternary mixtures of benzene-cyclohexane-toluene with dimethyl formamide (DMF) and N-methyl-2-pyrrolidone (NMP) were studied using Aspen Plus and PRO/II simulators. The Aspen Plus built-in binary interaction parameters for the toluene-DMF, benzene-NMP and cyclohexane-NMP systems resulted in inaccurate phase behavior calculations. The vapor-liquid equilibrium (VLE) for the three binary systems was regressed to illustrate the importance of using accurate model parameters. The obtained binary interaction parameters described the phase behavior more accurately compared with the built-in binary interaction parameters in Aspen Plus. In this study, the effects of the regressed and built-in binary interaction parameters on the ED process design are presented. The total annual cost (TAC) was calculated to further illustrate the importance of the regressed binary interaction parameters. The results show that phase behavior and thermodynamic model parameters should receive more attention during the research and development of ED processes. Distillation1, which is based on the relative volatility differences between components in a mixture, is one of the most important separation technologies. However, it is difficult to separate mixtures efficiently using con- ventional distillation when they have similar boiling points or form azeotropes. To separate azeotropes, several advanced distillation technologies have been studied, such as pressure-swing distillation2–7, azeotropic distilla- tion8, 9 and extractive distillation (ED)10–15. ED is achieved by adding an appropriate solvent with a higher boiling point to enhance the relative volatility of the components. ED is one of the most economical ways to separate close-boiling mixtures and has been widely used in the chemical and petroleum industries16. The design and optimization of ED is becoming increasingly attractive due to its potential economic advantages. The effectiveness of an ED process relies on the proper choice of sol- vent17. The solvent should be easy to recover and possess a high thermal stability, low toxicity, and high boiling point18. The key method for solvent selection is to compare the changes in the degree of relative volatility after adding different solvents. For example, dimethyl sulfoxide is a hydrogen bond breaker that is effective in break- ing the azeotropes of tetrahydrofuran-water18 and trimethyl borate-methanol19, the solvent butyl propionate can greatly enlarge the relative volatility of isobutyl acetate and isobutyl alcohol because the solvent forms a homol- ogous series with isobutyl acetate20, and N-methyl-2-pyrrolidone (NMP) was selected as a suitable solvent for n-heptane-isobutanol separation21. Residue curve maps (RCMs) can also be used to find suitable solvents for separation processes. Based on an RCM analysis, water was used as the solvent to separate acetone-methanol22, and the nontoxic solvent tetraethyleneglycol was used for ethanol-water separation since distillation boundaries did not appear in the RCM23. Optimization and process intensification are the two other factors that should be considered when designing an economic ED process. To reduce the total annual cost (TAC) and achieve further energy saving, methods for process intensification and integration combined with ED have been published in many papers24–31 using ther- mally coupled distillation column25, dividing-wall column26 and double-effect distillation31 techniques. Many College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China. Correspondence and requests for materials should be addressed to Y.W. (email: [email protected]) SCIENTIFIC REPORTS | 7: 9497 | DOI:10.1038/s41598-017-09088-2 1 www.nature.com/scientificreports/ Figure 1. T-xy diagram of the Toluene-DMF mixture using the built-in interaction parameters at 1 atm: NRTL model; UNIQUAC model; and Wilson model. Figure 2. Optimization algorithm interfaces for the Extractive Distillation Optimization Software. simulation platforms, such as Aspen Plus, HYSYS, PRO/II and ChemCAD, have been used for the conceptual design of extractive distillation, and achievements have been made to improve the economics and controllability. Luyben32 provided a detailed introduction for creating a steady-state design and optimizing and assessing the controllability of a distillation process using Aspen Plus. Long and Lee33 investigated the ED process with a retro- fit design using HYSYS to achieve further energy saving and to improve the process capacity. Shirsatet al.34 rig- orously optimized an ED configuration using ChemCAD, and found that the process of ethanol dehydration was more economical using a pre-separator column. Timoshenko et al.25 investigated several alternative ED processes with and without partially thermally coupled columns for different types of ternary mixtures, and a case study was discussed to select the most energy-saving process using PRO/II software. All of the above studies promoted the development of ED. In the separation process, the selection of the thermodynamic model in the simulator software is a primary issue for performing the phase equilibrium calculation35–38. Dimethyl formamide (DMF) and NMP are widely used as solvents to design ED processes, and the unusual phase behaviors caused by the different interaction parameters of toluene-DMF, benzene-NMP and cyclohexane-NMP attracted our interest. Mixtures of DMF and nonaromatics, such as cyclohexane and heptane, can form minimum boiling point azeotropes39. Luyben40 selected DMF as the solvent to design an ED process using the built-in binary interaction parameters of the NRTL model. The calculation with the built-in binary interaction parameters for toluene-DMF in Aspen Plus, however, indicated that the mixture forms a homogeneous minimum boiling azeotrope at atmospheric pressure (Fig. 1). As seen from the experimental data published in Azeotropic Data41 at different pressures, the binary system of toluene-DMF does not exhibit azeotropic behavior. NMP has been chosen as a suitable solvent for separating aromatic and nonaromatic mixtures42–47. Methods for solvent selection have demonstrated that NMP efficiently alters the relative volatility of benzene-cyclohexane42–44. Vega et al.43 found that NMP is an efficient solvent for separating benzene-cyclohexane-cyclohexene mixtures using non-steady-state gas chromatography. However, a few studies40, 48 have shown that NMP is not an efficient solvent for separating mixtures that contain binary azeotropes of benzene-cyclohexane. The purpose of this article was to study the effect of various thermodynamic model parameters when design- ing ED processes for separating mixtures using DMF and NMP solvents. The VLE data that have been reported in the literature49, 50 were regressed to obtain the binary interaction parameters in order to demonstrate that the binary system of toluene-DMF does not exhibit azeotropic behavior and that NMP can be used as an appropriate solvent for separating azeotropic mixtures of benzene-cyclohexane during ED. The detailed separation process was carried out using the regressed binary interaction parameters to obtain accurate separation results. SCIENTIFIC REPORTS | 7: 9497 | DOI:10.1038/s41598-017-09088-2 2 www.nature.com/scientificreports/ a a Model aij aji bij/K bji/K αij RMSD (P/kPa) RMSD (yi) Toluene (i) + DMF (j) NRTLb 0 0 332.814 −28.501 0.10 0.140 0.0119 UNIQUACc −7.297 7.608 2683.662 −2884.069 0.168 0.0118 Wilsond 0 0 −125.353 −200.774 0.145 0.0117 Benzene (i) + NMP (j) NRTLb 0 0 3776.060 −2904.104 0.03 0.026 0.0004 UNIQUACc −1.510 1.088 666.906 −528.010 0.089 0.0010 Wilsond 2.386 −2.442 −1103.648 1121.244 0.076 0.0007 Cyclohexane (i) + NMP (j) NRTLb 0 0 1085.790 −270.811 0.14 0.027 0.0002 UNIQUACc 9.229 21.900 −3414.744 −7217.892 0.066 0.0001 Wilsond 0 0 −181.280 −769.390 0.045 0.0004 Table 1. Correlated model parameters and average absolute deviations for the systems. 1 aRMSD = 1 ∑n (Mexp − M)cal22 , where n is the total number of data points and Mexp and Mcal represent the n i1= i i b c experimental and calculated values, respectively. NRTL model: τij = aij + bij/T. UNIQUAC model: d τij = exp(aij + bij/T). Wilson model: In Aij = aij + bij/T. Figure 3. Comparison between the experimental data, correlated results and default results for the system of Toluene-DMF at T = 373.15 K: experimental data; correlated results from the NRTL model; correlated results from the UNIQUAC model; correlated results from the Wilson model; and the default results from the NRTL model. Methods Data regression. Experimental data were obtained using the function from the National Institute of Standards and Technology ThermoData Engine (NIST TDE) in Aspen Plus V8.8. The NRTL, UNIQUAC, and Wilson models were used to correlate the experimental data for this binary system. Area consistency tests were used to check the thermodynamic consistency. Experimental data49, 50 for the systems of toluene-DMF, ben- zene-NMP and cyclohexane-NMP, which passed the thermodynamic area consistency test, were regressed to obtain binary interaction parameters. The root mean square deviation51, 52 (RMSD) values were calculated between the experimental and calculated results to select the appropriate thermodynamic model. The non-ran- domness parameter (α) of the NRTL model was set at different values. Renon and Prausnitz53 recommended 54, 55 setting α between 0.2 and 0.5. In many examples in the literature , the values of αij were smaller than 0.2 to achieve better regression results. A wider interval for α was considered to correlate the experimental data with high accuracy, and the value of α was optimized by minimizing the RMSD. Economics. The TAC is the sum of the annualized operating costs and capital costs, and the relevant approx- imation methods were taken from Douglas56.